The present invention generally relates to hardness measuring and more particularly, to a method of measuring hardness of a flexible thread-wound golf ball or its inner core portion wound with flexible thread.
Commonly, the constructions of golf balls may be broadly divided into two types, i.e., one being the so-called thread-wound golf ball comprising a core portion of round shape, a flexible thread, for example, of rubber tightly wound around said core portion under the application of a tension to a certain extent to form a thread-wound inner core of the thread-wound golf ball, and an outer layer, for example, of gutta percha, plastics and the like applied onto said flexible thread for covering, and the other being the so-called solid golf ball which is made through one-piece molding, by subjecting to vulcanization during heating under pressure, a mixture including rubber, crosslinking resin, polymerization initiator and filling agent, etc. and filled in a mold. The present invention relates to a method of measuring the hardness of the thread-wound golf ball or its inner core portion wound with the flexible thread.
Although there has been no established practice for a method of measuring the hardness (sometimes referred to as "compression" by those skilled in the art) of the thread-wound golf ball or its inner core portion wound with the flexible thread so far, conventional methods normally employed therefor may be broadly classified into the following two procedures.
(i) To represent the hardness by a reaction force produced when the golf ball or its inner core is subjected to a strain by a predetermined amount. PA1 (ii) To represent the hardness by the amount of strain when a predetermined amount of load is applied to the golf ball or its inner core.
In both of the above conventional methods of hardness measuring, the strain to be imparted and the amount of the strain consequently produced are limited only to about 5% of the maximum diameters of the golf ball and its inner core to be measured, and moreover, since the measured values obtained in the above case are markedly influenced by non-uniformity of the winding density of the flexible thread on the inner core or of elasticity modulus of the flexible thread, there is such an inconvenience that the hardness differs from spot to spot even when measurements are taken on one golf ball or its inner core, thus showing the so-called anisotropy, and the deviations in the measured hardness resulting from the anisotropy reaches as high as 10 to 20%, presenting serious problems in the hardness control of the golf balls.
On the other hand, the strain to be imparted to the golf ball during impact in actual play is by far larger than the strain produced in the hardness measurement of the golf ball, and normally amounts to 20 to 30% of the diameter of the golf ball. The hardness of the ball to be felt by the hand or "stony feeling" in the above case is not a local hardness as obtained by the conventional measuring methods, but is considered to be one closer to the average value of the hardnesses obtained on all portions of one golf ball.
Incidentally, since the hardness of the golf ball of the above described type is produced by winding the flexible thread, for example, rubber thread onto the core portion in a stretched state, the average values of the hardnesses as described above is thought to be in proportion to the energy possessed by the wound rubber thread. Therefore, if the energy possessed by the wound rubber thread can be measured by some means, more realistic or practical hardness indication with small deviations may be achieved.
With particular attention directed to the above point, the present inventors have made earnest investigations into such means as described above, and finally, come to think of utilization of ultrasonic waves for the purpose.